Fiber-reinforced moldings may be produced by known processes using a number of different synthetic polymers. Glass fibers have proven to be a particularly advantageous reinforcing material. It is known that storage stable, semifinished products such as resin mats, prepregs and molding compositions can be produced from resins containing unsaturated polyesters (hereinafter referred to as "UP resins") and glass fibers. The semifinished products thus obtained may be formed by means of heated presses and subsequently hardened to form moldings of very high strength and rigidity. Unfortunately, this process involves numerous disadvantages. The vinyl group-containing monomers, for example styrene normally used as solvent for the UP resins, give rise during the radically initiated polymerization reaction to a very high density of cross-linking which, in turn, makes the molding extremely brittle and sensitive to notched impact. In addition, the presence of solvents makes processing more complicated because elaborate measures have to be taken to extract the solvent vapors, in addition to which explosion-proof processing installations have to be used.
So-called "monolayer laminates" consisting of a sheet-form glass fiber formation and epoxide resins as binder are widely used, for example in the ski industry, for the production of sandwich elements.
In addition, however, thicker laminates consisting of several layers of a glass cloth impregnated with a thermosetting binder are also required. Hitherto, "laminates" such as these have also been produced with epoxide resins as binder. With certain hardeners, generally aromatic amines or dicyanodiamide, epoxide resins form a "B-stage", i.e. an intermediate stage in which the originally liquid resin-hardener mixture has solidified but not completely hardened. In this intermediate stage, the resin is brittle, friable and may still be dissolved in a solvent (for example, acetone) at room temperature and also remelted at elevated temperature, for example 160.degree. C. In this B-stage, the resin is partly crosslinked and still thermoplastic and, in addition, still contains free epoxide groups and amino groups. By relatively gentle heating, the remaining epoxide and amino groups are also reacted with one another so that the resin gels. In this condition ("C-stage"), the resin is insoluble in standard solvents and also infusible.
Conventional polyurethane resins had never been known to behave in this way. The two-component systems of unmasked isocyanate and polyol which are used in practice all react fully in a single stage to form the duroplastic plastic. The reason for this lies in the considerable evolution of heat during hardening in the block and in the low dimensional stability under heat (glass transition temperature) of the duroplast obtained. Another reason why no attempts were made to produce storage stable intermediate products of the type in question was because of the sensitivity of the isocyanates to water. It was assumed that, in this "B-stage", the free isocyanate groups would react with atmospheric moisture as is the case, for example, with one-component lacquers and that the material would, therefore, show inadequate storage stability in the "B-stage".
However, the production of glass-fiber-reinforced polyurethane plastics is generally known and is described, for example, in German Offenlegungsschriften Nos. 2,164,381 and 2,014,899, U.S. Pat. No. 3,678,009 and P. H. Selden's standard work, "Glasfaserverstarkte Kunststoffe (Glass-Fiber-Reinforced Plastics)", Springer-Verlag, Berlin, 1967. Thus, glass-fiber-reinforced polyurethane moldings can be produced, for example, by injection molding or reaction injection molding. However, the mechanical properties, in particular the flexural strength, of moldings produced in this way can be increased to only a limited extent because, for processing reasons, the length of the fiber material must not be any greater than about 1 to 6 mm. One particular disadvantage lies in the fact that on account of the limited fiber length the coefficient of thermal expansion of the reinforced polyurethane elastomers is still several times higher than the expansion coefficient of steel.
German Pat. No. 968,566 describes a process for the production of high molecular weight cross-linked plastics in which an intermediate product is produced initially from a polyester containing hydroxyl groups, a glycol and a less than equivalent amount of diisocyanate and is subsequently reacted with an excess of a diisocyanate containing uretdione groups to form storage stable, semifinished products. Finally, these semifinished products may be subjected to plastic forming and hardened by the effect of heat to form elastic moldings.
The polyurethane elastomers of the present invention have a predominantly linear structure and, although tough and highly elastic, lack sufficient hardness and rigidity for numerous applications. The use of fibrous reinforcing material is not mentioned in German Pat. No. 968,566. Although glass fibers having a length of more than 6 mm may, in principle, be incorporated into the above-mentioned intermediate product, the high viscosity of the intermediate product would necessitate the use of mixing rolls or kneaders, of the type normally used for processing rubber, for mixing in the fibrous material and the uretdione diisocyanate required for cross-linking. However, if the fiber material were to be incorporated in this way, such intense shear forces would be generated that the individual fibers would be reduced to fractions of their original length so that the required effects of stiffening on the one hand and reducing the coefficient of thermal expansion on the other hand could no longer be fully obtained.
Glass-fiber-reinforced polyurethane-based sheet-form structures obtained by impregnating glass fiber mats with reaction mixtures of polyisocyanates and relatively high molecular weight polyhydroxyl compounds followed by hardening under heat are described in U.S. Pat. Nos. 3,061,497 and 3,730,936 and in British Pat. Nos. 1,139,114 and 1,226,843. The main disadvantage of all these processes lies in the fact that the moisture which always adheres to the glass fibers leads to bubble formation in the product by reaction with the polyisocyanate. Because of the foam formed around the fibers in this way, these layers are unsatisfactory in terms of their strength. This is particularly noticeable under dynamic stress. Resistance to alternating stress is poor. Another disadvantage lies in the fact that hardening generally has to be carried out under relatively high pressures.
The object of the present invention is to provide new, solvent-free, storage stable molding compositions which may be hardened at elevated temperature to form bubble-free moldings characterized by high rigidity, high impact strength and high dimensional stability.
This object is achieved by the molding compositions provided by the present invention.